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Keep it simple

Here's the secret of man's ability to speak clearly

No other animal can speak and sing like humans, and there is especially one reason for that: Evolution has simplified our larynx.

By Birgitte Svennevig, , 8/12/2022

It's hard to see something that isn't there. Perhaps this is it why it was only recently discovered that something is missing in the complex organ system that enables humans to speak: To be precise, we lack the vocal membranes that monkeys, and apes have.

- What allows us to speak and sing as clearly and stably as we do is this lack of vocal membranes, said Ole Næsbye Larsen, who is emeritus at the Department of Biology and studies animal sound communication, including how they produce sounds.

The discovery is surprising when you take into account that evolution often gets credit for specializing, fine-tuning or optimizing abilities by making anatomy and processes more complicated – but this time it is about simplifying.

Searching through anatomical collections

- It was a big surprise. We have been studying human anatomy for several hundred years and that of monkeys for quite some time, too, and scientists have completely missed this. But of course; it is difficult to spot something that is not there, said Ole Næsbye Larsen.

The research team initially became interested in vocal cords while studying larynxes of primates (monkeys and apes); in these, the vocal membrane sits on top of the vocal cords - but not in humans.

Therefore, they decided to comparatively and anatomically study larynxes from 44 species of monkeys and apes. Such preserved larynxes can be found in anatomical collections and were not difficult to find - but no one had ever thought to look systematically for vocal membranes.

It was a big surprise. We have been studying human anatomy for several hundred years and that of monkeys for quite some time, too, and scientists have completely missed this

Ole Næsbye Larsen, emeritus associate professor

- We found vocal membranes in all the larynxes. Also those from apes. But Homo sapiens, as the only primate species, did not have vocal membranes. On that basis, we conclude that evolution has removed the vocal membrane from humans at some point after we separated from our closest relative, the chimpanzee, six million years ago, explained Ole Næsbye Larsen.

Determining the function of the vocal membranes was a bigger challenge - but a fortunate coincidence made it possible:

On another occasion some 30 years ago, the Japanese zoologist Sugio Hayama had a chimpanzee on the operating table. Part of the examination was done with video endoscopy, while the monkey was under anesthesia. When waking up – a process of ca. 30 minutes – the monkey would occasionally grunt.

How we make sound:

Humans, mammals and birds produce sounds in the same way: the vocal cords are led into the airway and almost close it. When exhaled air from the lungs is forced up between the vocal cords, these are set into oscillations and produce sound, which is filtered by the alternating shape of the oral cavity and the tongue when we speak.

- Our Japanese colleagues found these old recordings and looked more closely at the movements of the chimpanzee’s vocal cords, and suddenly it dawned on them that the majority of the opening-closing movements was performed by the vocal membrane and not the vocal cords, as we had always thought!, said Ole Næsbye Larsen, continuing:

- Then we looked further and found some other 20 years old video endoscopy recordings, that I myself, together with German and Austrian colleagues, had from squirrel monkeys – and we found the same thing.

Monkeys produce more noise

The benefit of losing the vocal membranes was that our language became easier to understand.

Mathematical models, described in the paper, show that monkeys, who have vocal membranes, find it easier to produce powerful and high-frequency sounds than humans. On the other hand, their sounds easily turn into the kind of “noise" that we know from crying children, screams or the monkey house in the zoo.

- Humans have to spend a little more energy to produce a given sound when there is no vocal membrane. On the other hand, we do not produce "noise" and we get a much clearer and more stable voice, explained Ole Næsbye Larsen.

We are not necessarily the best communicators

Sure, the human ability to communicate using language is exceptional – but that doesn't mean we are the best communicators on the planet.

- There is still a lot we don't know when it comes to animal communication. For example, if you go for a walk with your dog, you will learn that it lives in a completely different universe, where it receives a lot of information from smells; but it is very difficult for us to imagine what it is like to be in a "smell universe", said Ole Næsbye Larsen.

- And think of octopuses; they produce color patterns that run across their body to communicate with others. We basically have no idea what it means or how complex their communication is. To that extent, there is still much to explore, he said.

About the study

The study was carried out by an international research team from Japan, Austria, Great Britain, Germany and Denmark. Takeshi Nishimura from Kyoto University in Japan led the study, Ole Næsbye Larsen from the Department of Biology is a co-author, and it is published in the journal Science. The study’s conclusion is based on anatomical studies, computer models and video endoscopy.

How to measure communication:

According to the mathematician Claude Shannon, the efficiency of communication can be measured as the number of bits one organism can transmit to another in one second. During a conversation, humans receive large amounts of bits of information from not only the spoken language, but also from tone of voice, facial expressions, body movements, etc. Other animals receive similar amounts of information from smells, vibrations, etc. around them.

Read also

Meet the researcher

Ole Næsbye Larsen is emeritus associate professor at the Department of Biology. He is a zoologist and studies various aspects of sound communication in birds (parrots and songbirds) and mammals (most recently walruses).


Editing was completed: 12.08.2022